This invention relates to communications systems, and, more particularly, to a material that reflects selected frequencies of microwave radiation.
Satellites are used to carry a large part of the private and government communications throughout the United States and the world. In the most basic satellite communications system, a ground transmitting antenna transmits a signal to a satellite receiving antenna mounted on a satellite in orbit above the earth. The signal is then transmitted by a transmitting antenna on the satellite to other spacecraft or to a ground receiving antenna. The signal can also be modified by the satellite, as by amplifying it before retransmission. Because the satellite is usually thousands of miles above the earth in a geostationary orbit, this procedure allows signals to be transmitted through the satellite to ground receiving stations thousands of miles from the ground transmitting station. This satellite transmission technique makes possible the beaming of television signals from central locations directly to local cable companies and backyard television dish receivers, as well as voice, data and other types of transmissions.
A major consideration in the design of satellite communications systems is the continuing need to reduce the weight of the components in orbit. The cost of raising one pound to geostationary orbit is thousands of dollars. Weight reduction anywhere in the system can be directly viewed in terms of reduced costs, or alternatively, in terms of added capability that may be furnished in place of the reduced weight.
Since the volume of satellite communications traffic is increasing, there is also a continuing need to provide more communications channels. This growth can be supplied either by launching more satellites, or by increasing the number of channels that can be carried by each satellite. Either of these alternatives may require the launching of additional pounds of hardware into orbit, at the high cost discussed previously, although the first alternative of launching more communications satellites would have a higher cost because of the need to duplicate systems already on orbiting satellites, such as the propulsion and guidance systems.
Most satellite communications are transmitted on microwave signals having frequencies of about 1000 megahertz and higher. To permit several different frequency ranges to be transmitted through a single antenna system, it has been proposed to provide a passive mechanical device that can be used to separate microwave signals of differing frequencies by selectively reflecting one frequency to a transceiver and passing other frequencies to further processing or to a transceiver. Such a device is termed a microwave frequency selective surface. The frequency selective surface would make possible the transmitting of a signal to the satellite having a number of different frequencies or channels. Each channel could then be selectively reflected out of the beam to its own processing electronics, obviating the need for separate electronics to perform the separation function. If the weight of the frequency selective surface were less than that of the electronics replaced, then there would be a net weight reduction of the satellite.
To make such a frequency selective surface feasible, the material used to construct it must have a number of characteristics which heretofore have not been available. The material should be formable into a flat or curved piece with sufficiently high modulus of elasticity and strength to hold its shape precisely during launch. The material should have a low coefficient of thermal expansion and expand generally isotropically, to minimize the effects of heating and cooling if the material is exposed to alternating direct sunlight and shade. When the material is heated by the sun, it expands. The expansion ideally would be negligibly small, so that uneven heating would not cause the frequency selective surface to wrap, which might degrade the microwave signal. Even if the device is heated evenly, anisotropic expansion can cause distortion.
The material of construction must have a surface finish that does not interfere with the signal. Since the wavelength of the signal may be as low as one-fifth of a thousandth of an inch, the surface of the frequency selective surface must be even smoother to avoid interference with the signal. The material of the body should have a low dielectric constant to minimize refraction of the signal by the device, and a low dissipation factor to minimize attenuation or signal loss by the body of the microwave frequency selective surface. The material of the body must permit application of any surface coatings or structures required to perform the frequency selection.
The material of the body of the microwave frequency selective surface should experience low outgassing in a space environment, since evolution of gas can interfere with operation of the satellite. It is also highly desirable that the material be stable to high temperatures to resist damage by intense beams of energy that might be directed against the surface, such as a high intensity laser beam that would burn holes in it or distort it. High temperature capability would allow the satellite to resist attack more effectively by lasers or other directed energy weapons, without the need for specialized shielding and defensive measures. Finally, it would be desirable to construct the device of a material having characteristics such as density, surface structure, and composition that can be varied over ranges to permit designers flexibility in their selection of frequency selective surfaces for different requirements, always using a basic material of construction with which they are familiar and for which data is readily available.
This combination of characteristics has not heretofore been available. Accordingly, there is a need for a material having the above properties for use in constructing the body of a microwave frequency selective surface. The present invention fulfills this need, and further provides related advantages.